bmips: add subtargets for each SoC

BMIPS is a generic arch that can be used for multiple Broadcom SoCs, each one
with its own specific drivers, so instead of having a huge kernel supporting
all of them, let's switch to a subtarget per SoC like other OpenWrt targets.

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>

target/linux/bmips/bcm6328/base-files/etc/board.d/02_network

@@ -0,0 +1,16 @@
+# SPDX-License-Identifier: GPL-2.0-or-later
+
+. /lib/functions/uci-defaults.sh
+
+board_config_update
+
+case "$(board_name)" in
+comtrend,ar-5387un)
+	ucidef_set_bridge_device switch
+	ucidef_set_interface_lan "lan1 lan2 lan3 lan4"
+	;;
+esac
+
+board_config_flush
+
+exit 0

target/linux/bmips/bcm6328/base-files/etc/uci-defaults/09_fix_crc

@@ -0,0 +1,11 @@
+# SPDX-License-Identifier: GPL-2.0-or-later
+
+. /lib/functions.sh
+
+case "$(board_name)" in
+comtrend,ar-5387un)
+	mtd fixtrx firmware
+	;;
+esac
+
+exit 0